Heating element



M. K. MURPHY HEATING ELEMENT Oct. 22, 1963 2 Sheets-Sheet 2 Filed Feb. 17, 1958 United States Patent O 3,108,170 HEATING ELEMENT Maxwell K. Murphy, 535 W. Goldengate, Detroit, Mich. Filed Feb. 17, 1958, Ser. No. 715,604 6 Claims. (Ci. MSL-19) This invention relates to electrical heating elements and particularly to radiant heating elements.

It has been proposed to make electrical radiant heating elements for space heating and other purposes by coating a metal sheet with vitreous enamel on both sides and fusing a metallic resistance gnid on the rear side. The resistance grid when connected across a voltage source will cause heating of the entire porcelainized sheet or panel, and infra red rays will be radiated from the front surface in a manner similar to the glass heating panels now in u-se.

It has been found that such porcelainized heating elements work well and are more ecient in heat radiating qualities than the glass elements, but difhculty in manufacture has prevented their use. The base of such porcelainized elements is preferably steel and the type of sheet steel suitable for use from the standpoint of cost, weight, rigidity and ability to adhere to the vitreous coating, has the characteristic of exuding gasprincipally hydrogenwhen heated. This type of steel is generally known in the trade as enameling iron and is perfectly satisfactory when coated on one surface only with vitreous enamel. When coated on both surfaces with vitreous enamel, the sheet steel can not breathe and cracking and spalling of the vitreous coating results. This cracking and spalling is fatal to the operation of the heating element because it causes interruption of the resistor circuit.

It is the object of my invention to provide an improved radiant heating element which eliminates the above mentioned difficulty but which retains all of the desirable heat radiating qualities of the so called porcelainized elements and of the glass elements.

Another object of my invention is to provide an improved radiant heating element which is more durable and rugged in use and more economical to produce than those now being manufactured.

Other objects and advantages of my invention will become apparent from the following specification which, taken in conjunction with the accompanying drawings, discloses preferred embodiments of my invention.

Referring to the drawings, in which reference characters have been used to designate like parts referred to herein:

FIG. l is a rear elevation of an element made in accordance with my invention and suitable for use in a space heating panel;

FIG. 2 is an enlarged fragmentary section taken along the line 2-2 of FIG. l;

FIG. 3 is a sectional view of a Water heater embodying my invention;

FIG. 4 is a perspective View of a bathtub embodying my invention;

FIG. 5 :is an enlarged sectional View taken on the line 5-5 of FIG. 4;

FIG. 6 is a part section taken along line 6-6 of FIG. 5, on a greatly enlarged scale; and

FIG. 7 is a modified version of FIG. 5.

Referring to FIGS. l and 2, it may be seen that l have shown a sheet l0 of metal. Steel, preferably enameling iron, is preferred, but any suitable metal to which a vitreous coating may be adhered can be used. Steel is preferred because of its rigidity per unit of weight and its heat transfer qualities.

The sheet l0 has holes 12 for mounting purposes. A vitreous coating 14 (FIG. 2) is applied to the front or radiating surface of the sheet '10 by any of the methods now in common use in the industry. The coating may be white, or colored frit may be used to provide any desired decorative effect.

The vitreous coating ld may be of almost any desired thickness, a thickness of one-sixteenth of an inch having been found satisfactory. On the rear surface of the sheet l0, a coating 16 of resin is applied. The thickness of this coating may vary from a very `thin layer in the order of .0l inch to .125 inch depending upon the voltage intended to be used across the element. The coating 16 is of a resin having excellent electrical insulating characteristics and which will not be affected by heat at least within the range of operation of the element.

A suitable resin is available in the trade under the trade name Teflon 10U-X, from E. I. du Pont de Nemours & Co. This resin is a perfluocarbon resin With high dielectric characteristics at high temperatures and will stand continuous service at 400 F. Furthermore, this resin is odorless when hot, has suicient elasticity to withstand expansion and contraction of the sheet during its cycles of heating and cooling without cracking or peeling, and has good abrasion resistance.

Other resins and substances may be used for the coating lo such as resins manufactured by the Bakelite Division of Union Carbide Corp. and silicone varnishes manufactured by Dow-Corning Company, it being necessary only that the substance used have the insulating, heat resistant, expansion and contraction, and abrasion qualities set forth above. The Teflon 10G-X resin appears to be the most suitable.

The resistor i8 preferably consists of aluminum or aluminum alloy applied by means of spraying such as by a Schoop gun. The metal is applied in the molten state and adheres to the resin coating 16 upon cooling. Some improvement in adherence may be obtained by heating the coated sheet to a point where the resin coating 16 is slightly plastic during the metal spraying operation, but such is not necessary in ordinary instances. The resistor I8 may be applied as a labyrinth or grid by spraying through a mask, or preferably, the entire surface 16 is coated with metal, then the sheet is acid-etched to provide the desired pattern as described below.

The sheet, coated with the layers .14, 16 and 18, is next coated with a layer of silicone varnish through a silk screen such that the desired grid des-ign of resistor will be obtained in an etching bath which will etch away the metal not covered by the silicone varnish. The sheet is then baked or air dried to set the varnish and placed in an etching bath.

Suitable varnishes are #996 or #803 silicone Varnishes manufactured and sold by Dow-Corning Company and commercially obtainable. Both have good heat resisting properties and are acid resistant. #996 requires baking at about 250 C. to set it, but #803 may be air dried.

With aluminum or aluminum alloy as a resistor and either of the varnishes mentioned, an etching solution of dilute hydrochloric acid may be used. A 30% sodium hydroxide solution heated to 70 C. is also suitable. The bath will etch away the uncoated metal and leave a sharply dened grid design like that shown in FIG. 1.

The varnish coating 20 remains on the resistor and serves another useful purpose in that it prevents corrosion of the resistor which might occur from exposure to a corrosive atmosphere. Corrosion or oxidation of the resistor bars tends to cause localized high resistance spots which, in turn, cause localized hot spots resulting in failure of the element.

Terminals Z2 are attached to the resistor by soldering or the like, and upon connection of the terminals across a power source of suitable voltage, the heating element will heat quickly and uniformly and will radiate rays mostly in 3 the infra red portion of the spectrum from its front vitreous surface 14.

lt desired, a final protective coating of resin or silicone varnish may be appiied over the entire rear surface of the element, but this is not necessary for ordinary uses such as home heating, etc.

Reference is now made to FIG. 3 which shows the application of my invention to a Water heater of the well known domestic type.

The tank 24 of the well known and Widely advertised glass lined type is encased in an outer casing l26, heat insulating material 28 such as iiberglass or the like being disposed around the tank. An inlet 30 and outlet 32 is provided.

The so called glass lined water tanks are actually coated with porcelain or vitreous substance on the inside surface, and the outer surface is ideal for application of a heating element such as above described in connection with FGS. 1 and 2.

The resistors 34 and 36 are applied to the tank in the saine manner described, which description will not be repeated in the interest of brevity. It has been found that application of such a radiant heating element directly to the outer surface of a water tank instead of using a strap heating element or an immersion element as is common in the art, results in a substantial improvement in eiiiciency and rapidity of heating besides eiiecting a great economy in manufacturing of the tank.

While I have shown two heating resistors 34 and 36, it will be understood that one or more than two may be used and they may be rdisposed in any desired location on the outer surface of the tank 24.

FlGS. 4F6 inclusive illustrate application of the invention to a bathtub. The tub 38 is of the ordinary domestic type with a steel core covered on its exposed surface with vitreous material 48.

apply a resinous layer 42, a metallic resistor 44 and a silicone varnish layer 46 (FIG. 6) to portions of the tub, as shown in FiG. 5, in the manner described above. I have shown three heating elements on the tub in FIG. 5, but the bottom or side elements may be omitted depending upon what degree of heating is desired.

By switching on the bathtub heaters before the tub is lled with Water, a warrn and pleasant to touch tub results, and the bathwater may be maintained at any desired temperature for an indelinite period. It is understood, of course, that suitable thermostatic temperature control is provided for the tub, as well as for the water heater and the space heater above described.

The bathtub 38 when constructed as shown also serves as a space heater for the bathroom. The considerable mass of steel and porcelain when heated will radiate considerable heat and usually enough to heat a Ismall bathroom. in cold climates, it may be desirable to position the side mounted element on the inner surface of the bathtub wall that is usually exposed to the room, thereby to provide direct radiation to the space.

FIG. 7 illustrates such a modification. In this version, the element may be in addition to one or more of those shown in FIG. 6, or it may be the sole element.

A bathtub with a built-in heater is useful when bathing children and is especially useful for invalids or arthritic persons who must take hot baths for therapeutic purposes.

Any suitable or available voltage may be used with my improved heating element, the ordinary 220-240 volt household circuit being preferred. Higher voltages may be used when necessary, and 110 volts is suitable where 220240 volt lines are not readily available. The resistance of the resistor grid and the thickness of the plastic layer separating the resistor from the metal plate must be properly chosen to give the performance characteristics desired, and these values are readily calculated by ones skilled in the art.

While l have shown for illustrative purposes only preferred embodiments of my invention, l wish it understood that I do not thereby intend to limit the practice or scope of the invention except as defined by the appended claims.

l claim:

1. A sheet type electric resistance radiant heater adapted for wall mounting comprising a metal plate having a vitreous coating on the radiating side thereof, the opposite side of said plate having a layer of periluorocarbon resin of dielectric and heat resistant characteristics adhered thereto with a metallic resistance grid adhered to the exposed surface of said resin layer.

2. The combination of claim 1 wherein said plastic consists of a resin capable of withstanding a continuous temperature of at least 400 F.

3. A water heater comprising a tank lined with vitreous substance, a layer of dielectric, heat resistant resin on a selected portion of the outside of said tank, and a metallic resistor grid in the form of a thin, ilat layer adhered to the exposed surface of said resin layer.

4. A water heater comprising a vitreous lined tank, a layer of dielectric, heat-resistant resin adhered to the outer side of said tank, a metallic resistor layer deposited on the exposed surface of said resin layer and means for connecting said resistor layer to a power source, whereby heat generated by said resistor is radiated directly into the water in said tank.

5. The combination set forth in claim 4 wherein said resin layer comprises a perlluorocarbon resin.

6. The combination set forth in claim 4 wherein said resin layer comprises a perfluorocarbon resin capable of withstanding a continuous temperature of at least 400 F.

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1. A SHEET TYPE ELECTRIC RESISTANCE RADIANT HEATER ADAPTED FOR WALL MOUNTING COMPRISING A METL PLATE HAVING A VITREOUS COATING ON THE RADIATING SIDE THEREOF, THE OPPOSITE SIDE OF SAID PLATE HAVING A LAYER OF PERFLUOROCARBON RESIN OF DIELECTRIC AND HEAT RESISTANT CHARACTERISTICS ADHERED THERETO WITH A METALLIC RESISTANCE GRID ADHERED TO THE EXPOSED SURFACE OF SAID RESIN LAYER. 